Parameters

Refers to the host command-queue in which the read command will be
queued. command_queue and image
must be created with the same OpenCL context.

image

Refers to a valid image or image array object.

blocking_read

Indicates if the read operations are blocking or
non-blocking.

If blocking_read is CL_TRUE
i.e. the read command is blocking, clEnqueueReadImage
does not return until the buffer data has been read and copied into memory
pointed to by ptr.

If blocking_read is CL_FALSE
i.e. map operation is non-blocking, clEnqueueReadImage
queues a non-blocking read command and returns. The contents of the
buffer that ptr points to cannot be used until the read
command has completed. The event argument returns an
event object which can be used to query the execution status of the read
command. When the read command has completed, the contents of the buffer
that ptr points to can be used by the application.

origin

Defines the (x, y, z) offset in pixels in the 1D, 2D, or
3D image, the (x, y) offset and the image index in the
image array or the (x) offset and the image index in the 1D image array. If
image is a 2D image object, origin[2]
must be 0. If image is a 1D image or 1D image buffer
object, origin[1] and origin[2]
must be 0. If image is a 1D image array object,
origin[2] must be 0. If image
is a 1D image array object, origin[1] describes the
image index in the 1D image array. If image is a 2D
image array object, origin[2] describes the image
index in the 2D image array.

region

Defines the (width, height, depth) in pixels of the
1D, 2D or 3D rectangle, the (width, height) in pixels
of the 2D rectangle and the number of images of a 2D image array or the
(width) in pixels of the 1D rectangle and the number
of images of a 1D image array. If image is a 2D image
object, region[2] must be 1. If image
is a 1D image or 1D image buffer object, region[1]
and region[2] must be 1. If image is a 1D image array
object, region[2] must be 1. The values in region cannot be 0.

row_pitch

The length of each row in bytes. This value must be greater than or
equal to the element size in bytes * width. If
row_pitch is set to 0, the appropriate row pitch
is calculated based on the size of each element in bytes multiplied by
width.

slice_pitch

Size in bytes of the 2D slice of the 3D region of a 3D image or
each image of a 1D or 2D image array being read. This must be 0 if
image is a 1D or 2D image. Otherwise this value must be greater
than or equal to row_pitch * height.
If slice_pitch is set to 0, the appropriate slice
pitch is calculated based on the row_pitch *
height.

ptr

The pointer to a buffer in host memory where image data is to be read from.

event_wait_list
,
num_events_in_wait_list

Specify events that need to complete before this particular
command can be executed. If event_wait_list
is NULL, then this particular command does not wait on any
event to complete. If event_wait_list is
NULL, num_events_in_wait_list must be 0. If
event_wait_list is not NULL, the list of events
pointed to by event_wait_list must be valid
and num_events_in_wait_list must be greater
than 0. The events specified in event_wait_list
act as synchronization points. The context associated with events in
event_wait_list and command_queue must
be the same. The memory associated with event_wait_list
can be reused or freed after the function returns.

event

Returns an event object that identifies this particular read command
and can be used to query or queue a wait for this particular command
to complete. event can be NULL in which case
it will not be possible for the application to query the status of
this command or queue a wait for this command to complete. If the
event_wait_list and the event
arguments are not NULL, the event argument should not
refer to an element of the event_wait_list array.

Notes

Calling clEnqueueReadImage to read a region of the
image with the ptr argument value set to
host_ptr + (origin[2] * image
slice pitch + origin[1] * image
row pitch + origin[0] * bytes per
pixel), where host_ptr is a pointer to the memory
region specified when the image being read is created with
CL_MEM_USE_HOST_PTR, must meet the following requirements in
order to avoid undefined behavior:

All commands that use this image object have finished execution before the read
command begins execution.

The row_pitch and slice_pitch argument
values in clEnqueueReadImage must be set to the image row
pitch and slice pitch.

The image object is not mapped.

The image object is not used by any command-queue until the read command has
finished execution.

If the mipmap extensions are enabled with cl_khr_mipmap_image,
calls to clEnqueueReadImage,
clEnqueueWriteImage and
clEnqueueMapImage can be used to read
from or write to a specific mip-level of a mip-mapped image.
If image argument is a 1D image, origin[1] specifies the mip-level to use.
If image argument is a 1D image array, origin[2] specifies the mip-level to use.
If image argument is a 2D image, origin[3] specifies the mip-level to use.
If image argument is a 2D image array or a 3D image, origin[3] specifies the mip-level to use.

Errors

clEnqueueReadImage returns CL_SUCCESS if
the function is executed successfully. Otherwise, it returns one of the following errors.

CL_INVALID_COMMAND_QUEUE if command_queue
is not a valid host command-queue.

CL_INVALID_CONTEXT if the context associated with
command_queue and image are not the same
or if the context associated with command_queue and events in
event_wait_list are not the same.

CL_INVALID_MEM_OBJECT if image is not
a valid image object.

CL_INVALID_VALUE if the region being read specified by
origin and region is out of bounds or if
ptr is a NULL value.

CL_INVALID_VALUE if values in origin
and region do not follow rules described in the argument
description for origin and region.

CL_INVALID_EVENT_WAIT_LIST
if event_wait_list is NULL and
num_events_in_wait_list > 0,
or event_wait_list is not NULL and
num_events_in_wait_list is 0, or if event objects in
event_wait_list are not valid events.

CL_IMAGE_FORMAT_NOT_SUPPORTED if image format (image channel
order and data type) for image are not supported by device
associated with queue.

CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to
allocate memory for data store associated with image.

CL_INVALID_OPERATION if the device associated
with command_queue does not support images
(i.e. CL_DEVICE_IMAGE_SUPPORT specified in
the table of allowed values for param_name for
clGetDeviceInfo
is CL_FALSE).

CL_INVALID_OPERATION if
clEnqueueReadImage is called on image
which has been created with CL_MEM_HOST_WRITE_ONLY or
CL_MEM_HOST_NO_ACCESS

CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST if the read
and write operations are blocking and the execution status of any of the events
in event_wait_list is a negative integer value.

CL_OUT_OF_RESOURCES if there is a failure to allocate
resources required by the OpenCL implementation on the device.

CL_OUT_OF_HOST_MEMORY if there is a failure to allocate
resources required by the OpenCL implementation on the host.